CLASSIFICATION OF MINEBAL LANDS. 77 



of 50 to 500 feet or more. Every case involving such conditions 

 must be considered on its merits, and here again the Geological 

 Survey has found it necessary to establish a series of general prece- 

 dents in order to make its treatment of such cases reasonably 

 uniform. In some places it has been found possible to treat a 

 group of coal beds as if they were all gathered at a "center of 

 weight " — ^that is, at a point determined by multiplying the com- 

 puted thickness of each bed by its depth from the surface and 

 dividing the sum of the products by the sum of the compensated 

 thicknesses, the thicknesses of the various beds being reduced for 

 partings in the manner already described. 



The determination of the limit m depth at which a coal bed can 

 be mined may be complicated by the fact that in a region of very 

 rugged topography the coal gets below minable limit in passing 

 under some high ridge or peak but is within the minable limit under 

 a valley on the other side. Again, in some places the coal outcrops 

 near the foot of a great cliff, such as commonly flanks a high plateau 

 in the West, and it goes below minable depth within a short distance 

 from its outcrop. In order to cover all the different occurrences of 

 this kind the regulations provide that tracts underlain by horizontal 

 beds of coal that lie at depths below the minable limit may under cer- 

 tain conditions be classified as coal land. Horizontal beds of coal may 

 be mined to a distance back from their outcrop equal to 10 times the 

 depth limit for coal of that particular grade and thickness. It is evi- 

 dent that, to be commercially minable at a certain depth, a coal bed 

 must be minable for a reasonable distance from the foot of a shaft 

 sunk to it, in order to pay the cost of the shaft. It is also evident 

 that if it is profitable to sink a shaft to the depth limit for a given 

 coal and then mine out horizontally by drift a certain distance from 

 tlie foot of the shaft, it would be profitable to mine farther from the 

 foot of the shaft if it were not so deep. The relative values involved 

 in shaft and drift can be easily calculated if unit values are fixed 

 for the horizontal and vertical components. For purposes of classi- 

 fication and valuation it has been assumed that 1 foot of vertical shaft 

 in rock involves as much expense in its construction and maintenance 

 and in the removal of the coal as 7.5 feet of horizontal drift in coal. 

 For this reason the horizontal distance to which the drift can be 

 carried will be no more than the prescribed limit for mining from 

 the outcrop (in this case set at 10 times the vertical limit) minus 

 7.5 times the depth of the shaft. Thus if the minable depth of a cer- 

 tain coal bed is 2,000 feet, but that bed is horizontal and crops out 

 in a cliff, the area underlain by it is classified as coal land for a dis- 

 tance of 10 times 2,000, or 20,000 feet, back from the outcrop. If the 

 bed is horizontal but lies at a depth of 700 feet, it is assumed that the 

 coal can be mined back 20,000 minus (700X7.5) =14,750 feet from the 



